Passive Intermodulation (PIM) occurs when signals are present in a passive device that exhibits some non-linear behavior. In a wireless communication device such as a base station of a wireless communication network, PIM occurs when a high power transmit signal is passed through a passive device that exhibits some non-linearity. This non-linear passive device is referred to as a PIM source. The PIM source may be a non-linear component in a transmit path of the wireless communication device such as, for example, a cable, a connector, a duplex filter, an antenna of the wireless communication device, or the like. The PIM source may alternatively be due to an object that is external to the wireless communication device (e.g., a fence). The wireless communication device may have multiple PIM sources.
The PIM created by a PIM source includes multiple Intermodulation Products (IMPs) (e.g., 2nd order, 3rd order, etc.) of the transmit signal. When any of the IMPs fall within a passband of a receiver of the wireless communication device, a resulting PIM distortion is introduced into the received signal and, as a result, the receiver is desensitized. PIM distortion is particularly problematic for multi-carrier or multi-band wireless communication devices. Multi-carrier or multi-band signals are an important characteristic of modern wireless communication standards (e.g., the Long Term Evolution (LTE) cellular communication standard) as well as multi-standard, or multi-band, wireless communication devices. As such, PIM distortion is becoming an increasingly important problem that needs to be addressed.
One previous approach to addressing PIM distortion is described in U.S. Patent Application Publication No. 2011/0075754, entitled “Mitigation of Transmitter Passive and Active Intermodulation Products in Real and Continuous Time in the Transmitter and Co-Located Receiver,” which was filed on Aug. 5, 2010 and published on Mar. 31, 2011. In this published patent application, PIM distortion is estimated by modeling the PIM distortion with a polynomial model in the digital domain at baseband. However, this polynomial model requires a very large number of polynomial orders to accurately model the PIM distortion. For example, in J. Henrie, A. Christianson, W. J. Chappell, “Prediction of passive intermodulation from coaxial connectors in microwave networks,” IEEE Trans. On Microwave Theory and Techniques, Vol. 56, No. 1, January 2008, pp. 209-216, it was determined that a 49th order polynomial was required to accurately model the AMFund−AMIM3 curve for the PIM of a Subminiature version A (SMA) connector, where AMFund−AMIM3 represents a relationship between the amplitude of the input signal (fundamental) to the amplitude of the output IM3 signal. The required amount of digital resources required for the polynomial model increases as the number of orders of the polynomial model increases. As such, an accurate polynomial model of the PIM distortion requires, in many cases, a prohibitively large amount of digital resources.
Another approach to compensating for PIM distortion is described in N.M. Amin and M. Weber, “Transmit and receive crosstalk cancellation,” 2010 6th International Conference on Emerging Technologies (ICET), Oct. 18-19, 2010, pp. 210-215. However, this approach assumes that the non-linear behavior of the PIM source (i.e., the shape of the AMFund−AMIM3 and AMFund−PMIM3 curves) is similar to that of the non-linearity of the power amplifier of the transmitter after power amplifier linearization has been applied. This assumption is very restrictive. As an example, a PIM source may have a certain ratio of 5th to 3rd order IMPs, but the linearized power amplifier may have a different ratio.
As such, there is a need for a system and method for compensating for PIM distortion in a receiver that overcomes the aforementioned problems associated with prior approaches.